Preoperative ionized calcium levels and risk of acute kidney injury after cardiac surgery

Background: Postoperative acute kidney injury (AKI) is a significant complication following cardiac surgery, with limited preventive measures.Aims: Our aim was to evaluate the association between preoperative serum ionized calcium levels and postoperative AKI in patients undergoing cardiac surgery.Methods: This retrospective cohort study included adult patients (≥18 years) who underwent elective or non-elective cardiac surgery at two tertiary hospitals between 2006 and 2020.Patients with an estimated glomerular filtration rate ≥15 ml/min/1.73m 2 who were not receiving maintenance dialysis were eligible.Preoperative serum ionized calcium levels were categorized 3 / 25 into quartiles (Q1-Q4).The primary outcome was postoperative AKI within 48 hours, defined according to Kidney Disease: Improving Global Outcomes criteria.Multivariable logistic regression and mediation analyses were performed.Results: Among 9779 patients (median age, 64 years; 60.1% male; 42.6% coronary artery bypass grafting, 57.4% valvular surgery with or without coronary artery bypass grafting; 4.6% emergency surgery; 10.0% chronic kidney disease stages 3-4), mild AKI (Kidney Disease: Improving Global Outcomes stage 1) and moderate-to-severe AKI (Kidney Disease: Improving Global Outcomes stage 2-3) occurred in 3848 (39.3%) and 719 (7.4%) patients, respectively.Acute kidney injury requiring dialysis occurred in 197 (2.0%) patients, and in-hospital mortality was 2.5%.Compared with Q4 (highest calcium), the adjusted odds ratios (95% confidence intervals) for mild AKI were 1.58 (1.38-1.80),1.21 (1.06-1.38),and 1.05 (0.92-1.20) for Q1, Q2, and Q3, respectively (P for trend <0.001).For moderate-to-severe AKI, the corresponding adjusted odds ratios were 2.11 (1.65-2.69),1.44 (1.12-1.87),and 1.16 (0.88-1.51) (P for trend <0.001).The association was stronger in patients with congestive heart failure and those undergoing prolonged cardiopulmonary bypass (both P for interaction <0.05).Mediation analyses suggested that intraoperative vasopressor use, cardiopulmonary bypass time, and red blood cell transfusion partially mediated the association between calcium levels and AKI outcomes.The lowest ionized calcium quartile was also independently associated with higher in-hospital mortality (Q1: adjusted odds ratio, 2.32; 95% confidence interval, 1.53-3.54).Conclusions: Lower preoperative serum ionized calcium levels were independently associated with higher risk of postoperative AKI following cardiac surgery.These findings suggest that ionized hypocalcemia may serve as a hemodynamic risk factor for AKI in this population.